1. Field of the Invention
This invention relates to a transformer having notch gaps extending partially across the flux path through the transformer core with a combined gap volume storing sufficient magnetic energy to substantially eliminate inductive spikes induced in the transformer secondary by a clipped sinusoidal voltage applied to the primary. The invention has particular application to apparatus for controlling the intensity of a low voltage lamp with a clipped sinusoidal voltage supplied through a step down transformer incorporating the notch gaps to eliminate filament ringing.
2. Background Information
A popular form of lighting today utilizes low voltage halogen and par (parabolic reflector) lamps. Maximum voltage for these lamps is 12 volts rms which requires a step down transformer for use with commercial power systems. Many of these lamps are controlled with a dimmer to provide a continuously variable level of light intensity. Filament ringing, that is, an audible sound produced by vibration of the filament in the lamp, is a common complaint with such lighting systems. Filament ringing from several lamps in the same room can be quite annoying. It also reduces the life of the lamp.
Filament ringing in these lighting systems can be traced to the inductive kick produced in the transformer voltage in response to the step changes in the clipped sinusoidal voltage supplied to the primary of the transformer by the dimmer circuit. These voltage spikes produce underdamped vibrations in the lamp filament which generate the annoying sound known as filament ringing.
A typical technique for eliminating voltage spikes in inductive circuits is to provide a choke in series with the load. The choke comprises a single winding, usually wound on a laminated core of magnetic material forming a closed flux path. Some chokes used to remove ripple from the output of rectifiers have gaps extending across the full width of the flux path in some of the laminations of the core so that those laminations do not become saturated by the dc current. An example of such a choke, also known as a retardation coil, is disclosed in U.S. Pat. No. 2,400,559. A type of choke known as a swing choke also incorporates gaps extending across the full width of the flux path at one or more locations to provide different values of reluctance at low and high loads.
It is known to provide gaps in at least some laminations of a transformer in order to fine tune the reluctance, since it is difficult to manufacture transformers with exactly the same reluctance. It is also desirable in some instances to provide transformers of the same general configuration but with varying values of reluctance. Gaps provided in the magnetic flux paths of such transformers to tune the reluctance, for the most part, extend across the full width of the flux path. Examples of such transformers can be found in U.S Pat. Nos. 1,606,755 and 1,606,761. Full width gaps are also used in the magnetic circuit of the transformer in U.S. Pat. No. 3,803,479 which is connected in series with a capacitor to form a resonant circuit which regulates the voltage on the secondary.
Gaps which extend the full width of the magnetic circuit significantly reduce the efficiency of a transformer. In addition, the increased current required in the primary to generate a desired secondary current raises the temperature of the transformer which further reduces its efficiency.
There remains, therefore, a need for a transformer which can substantially eliminate voltage spikes in a clipped sinusoidal voltage waveform applied to the primary.
There also remains a need for an efficient, simple means for substantially eliminating filament ringing in a low voltage lighting system energized by a clipped sinusoidal voltage waveform through a step down transformer.
More particularly, there remains a need for such a means in which the functions of stepping down the amplitude of the clipped sinusoidal signal and eliminating the voltage spikes are performed by a single component.